US8874391B2  Distancetofault measurement system capable of measuring complex reflection coefficients  Google Patents
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 G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
 G01R31/28—Testing of electronic circuits, e.g. by signal tracer
 G01R31/282—Testing of electronic circuits specially adapted for particular applications not provided for elsewhere
 G01R31/2822—Testing of electronic circuits specially adapted for particular applications not provided for elsewhere of microwave or radiofrequency circuits

 G—PHYSICS
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 G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
 G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
 G01R27/02—Measuring real or complex resistance, reactance, impedance, or other twopole characteristics derived therefrom, e.g. time constant
 G01R27/04—Measuring real or complex resistance, reactance, impedance, or other twopole characteristics derived therefrom, e.g. time constant in circuits having distributed constants, e.g. having very long conductors or involving high frequencies

 G—PHYSICS
 G01—MEASURING; TESTING
 G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
 G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
 G01R31/08—Locating faults in cables, transmission lines, or networks
 G01R31/11—Locating faults in cables, transmission lines, or networks using pulse reflection methods
Abstract
Description
This application is related to U.S. Pat. Nos. 7,061,221, 7,061,251, 7,171,322, and U.S. Pat. No. 7,254,511, each of which is herein incorporated by reference in its entirety.
The invention relates to techniques for measuring complex reflection coefficients of singleport radio frequency (RF) and microwave devices, and more particularly, to techniques for improving distancetofault (DTF) measurement systems to accurately measure complex reflection coefficients of singleport devices.
Distancetofault (DTF) generally refers to analyzing the performance of a communication medium, such as a transmission line or antenna. DTF typically employs time domain reflectometry (TDR) or frequency domain reflectometry (FDR), which allows for fault isolation in a transmission system by identifying the presence and location of signal path impairment (e.g., damaged transmission line or connector, loose connection, faulty component, etc). In short, such impairments can cause unwanted signal reflections, which in turn diminish the performance of the communication system. Thus, using DTF to identify impairments can improve system performance.
One effective way to characterize transmission systems, electrical networks, and other such linear networks is by scattering parameters (Sparameters), which can be obtained by measurements of a vector network analyzer. In this sense, a vector network analyzer can be used in carrying out DTF analysis. In particular, a vector network analyzer can be used to analyze magnitude and phase characteristics of transmission systems and electrical networks, which are characteristics associated with the reflection and transmission of electrical signals. A typical vector network analyzer can perform simultaneous transmission and reflection Sparameter measurements, complex impedance measurements, TDR and FDR measurements.
Unfortunately, vector network analyzers tend to be expensive and are generally bulky and heavy. There is a need, therefore, for cost effective and/or low bulk DTF techniques that allow for accurate measurement of complex reflection coefficients.
One embodiment of the present invention provides a method for computing a complex reflection coefficient of a unit under test (UUT) with a distance to fault (DTF) system having a probe that is operatively coupled to the UUT. The method includes performing probe alignment to produce measured alignment voltages, and adjusting calibration values based on the measured alignment voltages to produce adjusted calibration values. The method continues with measuring UUT voltages with the probe to produce measured UUT voltages, and computing a complex reflection coefficient of the UUT using the measured UUT voltages and the adjusted calibration values. The complex reflection coefficient is computed as a function of known scattering parameters of a multiport measurement circuit included in the probe. The method may include the preliminary steps of displaying a graphical user interface (GUI) on a display viewable by a user, thereby allowing a user to select desired test parameters of the DTF system, and modifying test parameters of the DTF system based on the user selections. The method may include displaying the computed complex reflection coefficient (e.g., graphically over a range of frequencies, or as one or more discrete values at corresponding frequencies). In one particular case, the scattering parameters of the multiport measurement circuit are mathematically represented as a 4 by 4 scattering matrix that takes into account detectors included in the probe. In one such case, the 4 by 4 scattering matrix completely specifies transfer functions of the probe's multiport measurement circuit, and is used in mathematically representing the DTF system and UUT to be: an expanded multiport measurement circuit (as a function of the complex reflection coefficients of the detectors included in the probe); an expanded source (as a function of an RF source attenuator setting); and an expanded UUT (as a function of switch paths and alignment standards). In one particular case, performing probe alignment to produce measured alignment voltages includes use of N alignment standards, such that for each alignment standard, voltages output by analogtodigital (A/D) converters of the probe at each of M frequencies are recorded, and adjusting calibration values based on the measured alignment voltages to produce adjusted calibration values includes using the voltages output by the A/D converters to mathematically adjust alignment of the calibration values. In another particular case, performing probe alignment to produce measured alignment voltages includes use of three alignment standards, such that for each alignment standard, voltages output by analogtodigital (A/D) converters of the probe at each of M frequencies are recorded. Here, the three alignment standards include an open circuit, a short circuit, and a 50 ohm broadband matched load. In another particular case, in computing the complex reflection coefficient of the UUT, detectors of the probe are modeled with a quadratic (polynomial order 2) equation and a sliding window (in frequency) is used to provide sufficient data to solve for quadratic coefficients required to model detector transfer function at each frequency of interest. In one such case, monotonicity is enforced on the detector transfer function. In another particular case, measuring UUT voltages with the probe to produce measured UUT voltages includes recording voltages output by analogtodigital (A/D) converters of the probe at each of M frequencies, and computing the complex reflection coefficient of the UUT using the measured UUT voltages and the adjusted calibration values includes using the voltages output by the A/D converters. In another particular case, computing the complex reflection coefficient of the UUT using the measured UUT voltages and the adjusted calibration values involves solving equations that are in the form of two intersecting circles, such that points of intersection represent possible reflection coefficient estimates, and the complex reflection coefficient is computed using the reflection coefficient estimates.
Another embodiment of the present invention provides a computer readable medium encoded with instructions that, when executed by a processor, cause a process to be carried out for computing a complex reflection coefficient of a UUT with a DTF system having a probe that is operatively coupled to the UUT. The process may be configured similarly to, or a variation of, the previously described method for a computing complex reflection coefficient. Another embodiment of the present invention provides a DTF system for computing a complex reflection coefficient of a UUT, the system having a probe that can be operatively coupled to the UUT. The system may be configured, for example with a number of modules that can be executed or otherwise function to carry out functionality similar to, or a variation of, the previously described method for computing a complex reflection coefficient.
The features and advantages described herein are not allinclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.
Techniques are disclosed for computing distancetofault (DTF) in communication systems. The techniques can be embodied, for instance, in a DTF system that provides a multiport probing device and DTF functionality, including computing distances to faults and the fault magnitudes. In addition, the DTF system is further configured with the ability to accurately measure complex reflection coefficients of the UUT, and/or return loss of the UUT. As is known, the return loss of the UUT is easily computed from the complex reflection coefficient (i.e., Return loss=1−reflection coefficient).
In order to allow the DTF system to accurately measure the complex reflection coefficient of the multiport probing device, the system is programmed or otherwise configured to take into account various probe scattering parameters (sparameters) that are ignored by conventional DTF systems. In particular, such conventional systems approximate these sparameters as being of zero magnitude and accept the resulting inaccuracies in the estimated distancestofaults. However, depending on the demands of the given application, measurements of complex reflection coefficients may require a higher level of accuracy in the probe scattering parameters, in order to obtain acceptable estimation accuracies. To this extent, and in accordance with one embodiment of the present invention, the DTF system is improved by eliminating the simplifying assumptions that certain scattering parameters in the 4port measuring circuit are zero, which in turn provides enough additional accuracy to allow the DTF system to accurately measure complex reflection coefficients.
Consider, for example, a test probe having a conventional 4port measurement circuit for analyzing a target system (generally referred to herein as unit under test, or UUT) in various DTF applications. One example such DTF system having a 4port measurement circuit operatively coupled to a UUT is shown in
As will be explained with further reference to
In more detail, and with respect to the expanded 4port measurement circuit, the complex reflection coefficients of the detectors are combined into the scattering matrix of the 4port measurement circuit, to provide the scattering matrix of the expanded 4port as shown in
In the intermediate view of the partially encapsulated DTF system shown in
The scattering matrix of the expanded 4port of
As can be seen in this equation, normal scattering matrix notation is used, with the b's being the voltage waves leaving the ports, and the a's being the voltage waves incident on the ports.
With further reference to
indicating that port 3 of the 4port measurement circuit is connected to port 5 of the detector reflection coefficient and port 4 of the 4port measurement circuit is connected to port 7 of the detector reflection coefficient. The partitioned matrix equation can be denoted in shorthand notation as:
Then, continuing with the matrix formulation procedure,
Note that Ŝ is the 4 by 4 scattering matrix of the expanded 4port, which includes the reflection characteristics of the diode detectors.
As will now be discussed with reference to
Then, the same type of matrix equation can be formed as previously (e.g., using the procedure outlined in, “The Junction Matrix in the Analysis of Scattering Networks,” by Shekel), with the a's and b's for the external ports in the top rows and the a's and b's for the internal ports in the bottom rows:
The connection information of the internal ports can be captured in a junction matrix, J, such that:
The partitioned matrix equation can be denoted in shorthand notation as:
Then,
Substituting in the scattering parameters of the expanded 4port:
Solving for the reflection coefficient of the expanded UUT:
where n=1, 3 or 4. However, no information is known about {circumflex over (b)}_{1 }(the voltage reflected by the expanded 4Port), so only the two equations involving {circumflex over (b)}_{3 }and {circumflex over (b)}_{4 }are useful. In addition, note that, in this equation, the magnitudes of {circumflex over (b)}_{3 }and {circumflex over (b)}_{4 }are measured relative to a_{1 }but not their phases (explicitly denoted by {circumflex over (θ)}_{n}), because the detectors are only capable of measuring amplitude.
The two equations can be set equal to each other (for n=3 and n=4) to allow for solving the two unknown phases, {circumflex over (θ)}_{3 }and {circumflex over (θ)}_{4}, as shown here:
This equation is in the form of two circles set equal to each other:
c _{3} +r _{3} e ^{jφ} ^{ 3 } =c _{4} +r _{4} e ^{jφ} ^{ 4 }
Then, the two intersection points (if they exist) can be found using conventional circlecircle intersection techniques. Once the two solutions for φ_{3 }or φ_{4 }are found, they can be plugged into these equations to solve for the two potential solutions for the reflection coefficient of the UUT, Ŝ_{55}.
In general, any Ŝ_{55 }magnitude greater than 1 is impossible, and the other solution should then be chosen as the correct one. If both solutions have magnitudes greater than 1 (due to measurement error) then one possible strategy is to pick the one that has the smaller magnitude. If the two circles do not intersect (due to measurement error), then the point halfway in the gap between the two circumferences can be chosen. Once the matrix of solutions is obtained (at some frequency points, there will be a single viable solution, at others there will be two), then one can interpolate through the frequencies containing single viable solutions and use this interpolated curve to eliminate the ambiguities for the frequencies with two viable solutions (as will be further discussed and demonstrated with reference to
As can be seen, the method begins with a number of manual steps carried out by the user, including powering on the DTF system and launching the complex reflection coefficient computation routine. Upon the user launching the complex reflection coefficient computation routine, the method further continues with automatically displaying a graphical user interface (GUI) on a display area viewable by the user. For embodiments where the processor is included in a laptop or other such portable computing device operatively coupled to the probe (e.g., via an Ethernet or other suitable connection), this display area may be, for example, an LCD display. Alternatively, for embodiments having an integrated processor within the probe itself, the display area may be a smaller LCD display configured into the probe housing. Alternatively, the user interface can be implemented without graphical display, as a set of switches that have positions associated with known parameter settings that allow the user to select a desired test parameter setup. In such a case, complex reflection coefficient computation results could be stored, for example, in a memory for later download into a computer.
The method of this example embodiment continues with the user selecting the desired test parameters. In the example case shown, the interface allows the user to specify the frequency sweep (including start and stop frequencies) as well as the sweep resolution (i.e., number of data points making up each sweep). The method further includes automatically modifying the test parameters based on the user's selections. The method continues with the user connecting the probe of the DTF system to the target UUT, and initiating the complex reflection coefficient computation routine by selecting the “Initiate Test” button on the GUI. Note that the modification of test parameters based on user selections can take place, for example, realtime as the user makes each selection. Alternatively, the modifications may take place at the point when all selections are made and the user initiates the complex reflection coefficient computation routine. Further note that the method may be configured to automatically initiate the complex reflection coefficient computation routine, for example, once it senses that the UUT is properly connected to the probe (e.g., based on an established resistance range associated with UUTs when properly connected).
In response to initiation of the complex reflection coefficient computation routine, the method continues with automatically performing probe alignment, so that the measured alignment voltages can be used to mathematically adjust calibration values (e.g., stored in calibration tables accessible to the probe) for better accuracy. This alignment procedure will be discussed in more detail with reference to
As can be seen, the probe alignment method of this example embodiment employs a nested loop, wherein for each of N alignment standards, the voltages output by the A/D converters of the probe (as shown in
In any case, as the next loop executes, the alignment method causes the switch (
The method continues with using the measured alignment voltages (the A/D converter output voltages measured during execution of the nested loop) to mathematically adjust alignment standard values stored calibration files (e.g., tables), thereby improving the accuracy of the stored calibration data. In more detail, a small error in the probe scattering parameters (e.g., measured on a microwave network analyzer and enshrined in calibration tables stored in probe memory) can result in a large error in the complex reflection coefficient estimate. These small errors are due to the normal inaccuracies inherent in all measurements, including those obtained with microwave network analyzers. To improve the complex reflection coefficient accuracy, the probe alignment method uses the realtime measurements output by the A/D converter (e.g., performed automatically as directed by the processor in the seconds just prior to the complex reflection coefficient measurement) from the alignment standards to adjust the stored scattering parameters so that the measured voltages conform to the computed ones, prior to using them to estimate the complex reflection coefficient of the UUT, which significantly improves the complex reflection coefficient accuracy.
As previously explained and further depicted in
Other enhancements to the alignment procedure will be apparent in light of this disclosure. For instance, conventional probe alignment algorithms generally assume a straight line detector transfer function. This straight line took the form of:
Output [Volts]=gain×input [dBm]+offset (for DLVA detectors)
Output [Volts]=gain×input [mW]+offset (for diode detectors)
However, as shown in
Because a quadratic equation has enough degrees of freedom to allow it to be nonmonotonic, a ‘make monotonic’ algorithm can be employed to enforce monotonicity on the detector transfer function, in accordance with an embodiment of the present invention. If the algorithm finds a section of the transfer function that has a different slope sign than the rest, it modifies data points so that all slopes have the same sign. For instance, the following example has one point in vector_in that is nonmonotonic (i.e., the point with the value 12.1 is the only point that is larger than the point to its right), so that line segment is the only place where the slope is negative:
vector_in=[0 2 4 8 12.1 12 14 16 18 20 22 24].
Now, processing the nonmonotonic vector through a make_monotonic function produces an output vector (vector_out) that is a modified monotonic version of the input vector (vector_in), so that the slope now is positive for every line segment.
vector_out=make_monotonic(vector_in)
vector_out=[0 2 4 8 10 12 14 16 18 20 22 24]
As can be seen, the make_monotonic algorithm is programmed or otherwise configured to replace the nonmonotonic point with the value 12.1 with a monotonic point with the value 10. The algorithm can be coded to select the closest monotonic point with which to make the replacement based, for instance, on the average distance between neighboring points in the vector.
As can be seen, the method for measuring UUT voltage of this example embodiment employs a loop, wherein the voltages output by the A/D converters of the probe (as shown in
In any case, the UUT measurement method of
A benefit of this complex reflection coefficient computation technique is a more intuitive detection statistic display that inherently removes the transmission line loss, where the magnitude of the detection statistic peak is truly the magnitude of the fault complex reflection coefficient, without a need to make a mathematical conversion (i.e., no IFFT). This improved computation is based on the realization that the equations to be solved are in the form of two intersecting circles. Two intersecting circles have at most two points of intersection, representing two possible complex reflection coefficient estimates. Valid complex reflection coefficient magnitudes are confined to the range of 0 to 1, so in many cases one of the potential solutions may be discarded because it violates this rule.
In cases where both potential solutions are in the range of 0 to 1, the complex reflection coefficient computation algorithm can be programmed or otherwise configured to choose the solution closest to the line computed by interpolating the solutions at nonambiguous frequencies to the ambiguous frequencies. This is demonstrated in
Other enhancements to the UUT measurement procedure will be apparent in light of this disclosure. For instance, one enhancement that can be used is video averaging to reduce residual alignment errors, which also improves complex reflection coefficient estimation accuracy. In more detail, there remains some zero mean residual complex reflection coefficient estimation error after performing the probe alignment. To reduce the effect of this error, video averaging can be used on the complex reflection coefficient estimate.
Other enhancements that may directly or indirectly impact the complex reflection coefficient estimate include tuning various algorithm parameters to improve complex reflection coefficient accuracy. For instance, example parameters that can be tuned or otherwise optimized include the size of the sliding window used in the probe alignment algorithm, the choice of the polynomial order used to model the detector transfer functions, the size of the complex reflection coefficient video averaging window, and the complex reflection coefficient detection threshold.
The probe alignment module is programmed or otherwise configured for performing probe alignment to produce measured alignment voltages. The cal adjust module is programmed or otherwise configured for adjusting calibration values based on the measured alignment voltages to produce adjusted calibration values. The measure UUT module is programmed or otherwise configured for measuring UUT voltages with the probe to produce measured UUT voltages. The compute complex reflection coefficient module is programmed or otherwise configured for computing a complex reflection coefficient of the UUT using the measured UUT voltages and the adjusted calibration values, wherein the complex reflection coefficient is computed as a function of known scattering parameters of a multiport measurement circuit included in the probe, wherein in computing the complex reflection coefficient of the UUT, detectors of the probe are modeled with a quadratic (polynomial order 2) equation and a sliding window (in frequency) is used to provide sufficient data to solve for quadratic coefficients required to model detector transfer function at each frequency of interest. The probe alignment module uses three alignment standards, such that for each alignment standard, voltages output by analogtodigital (A/D) converters of the probe at each of M frequencies are recorded. The three alignment standards including an open circuit, a short circuit, and a 50 ohm broadband matched load. The cal adjust module uses the voltages output by the A/D converters to mathematically adjust alignment of the calibration values. As previously explained, scattering parameters of the multiport measurement circuit can be mathematically represented as a 4 by 4 scattering matrix that takes into account detectors included in the probe. In such cases, the 4 by 4 scattering matrix completely specifies transfer functions of the probe's multiport measurement circuit, and is used in mathematically representing the DTF system and UUT to be: an expanded multiport measurement circuit (as a function of the complex reflection coefficients of the detectors included in the probe); an expanded source (as a function of an RF source attenuator setting); and an expanded UUT (as a function of switch paths and alignment standards). Monotonicity may be enforced on the detector transfer function. As previously explained, the compute complex reflection coefficient module can be configured to solve equations that are in the form of two intersecting circles, such that points of intersection represent possible reflection coefficient estimates, and the complex reflection coefficient is computed using the reflection coefficient estimates.
The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
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